Ken Dornstein was 19 years old and home for the holidays when he and his family received word that his 25-year-old brother, an aspiring writer living abroad, was one of 270 people killed when the New York-bound Boeing 747 he was flying on crashed in Lockerbie, Scotland.

Over the years Dornstein, who went on to become a documentary filmmaker, turned to his sleuthing skills to shake out critical details of one of the worst terrorist attacks against American citizens. He spent years trying to identify the man he believes made the bomb that took down Pan Am Flight 103 on Dec. 21, 1988.

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For Dornstein, 46, it’s been a nearly lifelong quest that began the day his brother David died, sending him on a Byzantine chase that included searching files kept by the FBI, the former East German police and ultimately to a jail in Libya.

The first step was coming up with a list of eight to 10 names of individuals from the files of the original investigation who seemed to play a role in the bombing but who had never been indicted.

“My idea was a pretty simple idea – to go with a list of names, start knocking on doors and find out who these persons were and would any of them tell me the truth,” Dornstein said.

In the summer of 2011, he set off for Libya.

The Somerville, Massachusetts, resident said he quickly found out that many of those named in the investigation were dead. Some had been killed before the revolution that led to the ouster of dictator Moammar Gadhafi. Some were killed during the revolution while others appeared to have been killed on orders from Gadhafi himself.

“By my count there were three people left alive who I believe played a role in it and who I was focused on trying to identify and find,” Dornstein said.

Of those the most intriguing was a mysterious figure who Dornstein said had been traveling with Abdel Baset al-Megrahi, the only person convicted in the attack, including on the day of the bombing.

“I thought if I could figure out who that person was, potentially that would make sense of the whole case.” Dornstein said.

Dornstein said he was eventually able to connect a passport number of the suspected explosives expert to a matching passport number linked by East German secret police to the bombing of a disco in Berlin a few years before Lockerbie.

For a long time Dornstein, and investigators, weren’t sure that the individual – a Libyan named Abu Agila Mas’ud – was even real.

“This is a guy who had essentially been a ghost. There was no photo of him. The FBI and Scottish police – nobody had a photo of him. There were suspicions that he didn’t even exist,” Dornstein said. “The Libyans always denied his existence.”

Dornstein said he was able to track down Mas’ud to a jail in Libya where he says he was serving a 10-plus year sentence on bomb-making charges for booby-trapping the cars of those opposed to Gadhafi.

“I wish I had come face to face to him,” said Dornstein, who only dealt with Mas’ud indirectly.

“I would have liked to have been there for the moment when someone tapped him on the shoulder and said ‘Hey, here’s this incident that happened all these years ago and there’s someone in America – someone’s brother – who said you connected the wires that maybe blew up that plane and killed some 270 people,” Dornstein said. “I would have loved to have been there to see his reaction and hear his answer.”

Prosecutors haven’t named the two Libyans, in keeping with the British practice of not identifying suspects until they’re charged. Dornstein said he doesn’t have any direct knowledge of the identity of the two, although he said he’s convinced Mas’ud is one.

British officials previously asked to interview a second man, Abdullah al-Senoussi, Gadhafi’s former spymaster, about the bombing. Abdullah al-Senoussi also was on Dornstein’s list. A third individual is still at large, he said.

Dornstein ultimately turned his search for the Pan Am Flight 103 bombers into a three-part investigation for the PBS “Frontline” documentary series.

The last of the three parts aired this week ahead of Thursday’s announcement.